In typical biological water treatment methods such as an activated sludge treatment method, aerobic microorganisms and denitrifying bacteria for eliminating nitrate nitrogen are often used.
When aerobic microorganisms are used, the inside of a treatment tank needs to be aerated with air or oxygen in order to maintain activity and improve treatment capacity of these microorganisms. In order to provide a constant treatment process when there is a variation in inflow load, various treatment methods using immobilized microorganisms for maintaining a high concentration of microorganisms in the treatment tank are used. Typical examples are a method of attaching microorganisms to the surface of a carrier material such as polyurethane, or a method of immobilization by entrapment of microorganisms within a carrier in which microorganisms are immobilized with polyvinyl alcohol and the like within a carrier.
Furthermore, when the denitrifying bacteria are used, a system of immobilizing a cell body by entrapment within polymer gel beads, and supplying a gas to be used for a reaction (a fluidized bioreactor) has been proposed.
As a typical example of carriers for immobilizing microorganisms, a hollow fiber membrane is proposed. If microorganisms are attached or immobilized to the surface of the hollow fiber membrane, the surface area of the membrane to be stored per unit volume can be enlarged. Accordingly, the concentration of microorganisms in the treatment tank is increased and also oxygen and the like which are necessary for treatment can be efficiently supplied, so that treatment efficiency can be improved.
As methods using a hollow fiber membrane, a method of nitrifying and denitrifying by propagating aerobic nitrifying bacteria to the surface of a hollow fiber membrane (for example, see patent references 1 and 2), or a method of immobilizing autotrophic bacteria which are able to reduce nitrate nitrogen to nitrogen gas within the hollow fiber to be come a bioreactor element (for example, see patent reference 3) are disclosed.
To treat the large amount of water to be treated by a bioreactor, a membrane having a large area is necessary and also water to be treated needs to make contact with microorganisms on the surface of membrane. Using a conventional module or bioreactor, there is a concern that treatment efficiency decreases if the amount of water to be treated exceeds an L/hr level. Furthermore, since installation space is limited, a compact sized module or bioreactor with a membrane having a large area is required.
Furthermore, if microorganisms attached on the surface of the hollow fiber membrane excessively propagate as the operation of a module or bioreactor continues, the microorganisms form some clumps between membranes; therefore, contact efficiency of clumps of microorganisms with water to be treated or gas supplied from the membrane decreases. This phenomenon remarkably appears as the membrane area is made larger and the number of hollow fiber membrane increases.
To prevent such a phenomenon, excess microorganisms or the like can be washed out from the surface of the membrane by means of air bubbling from the bottom of a module and the like. However, it is required that air bubbling be uniformly applied to the entire module; therefore, there is a problem in that uniformity in applying air bubbling decreases as the membrane area of the module increases.
Furthermore, as the membrane area increases, the large number of the hollow fiber membranes must be anchored with anchoring members. In this case, since the cross-sectional area which is perpendicular to the longitudinal direction of fibers provided in an anchored portion is enlarged, withstand pressure of the module against gas pressure to be supplied into the inside of the hollow fiber membrane decreases.    Patent reference 1: Japanese Unexamined Patent Application, First Publication No. H 10-85787    Patent reference 2: Japanese Unexamined Patent Application, First Publication No. 2000-218290    Patent reference 3: Japanese Unexamined Patent Application, First Publication No. 2003-33776